Cyanobacterial symbionts diverged in the late Cretaceous towards lineage-specific nitrogen fixation factories in single-celled phytoplankton

• Published in: Nature Communications
• Main Authors: Cornejo-Castillo, F.M., Cabello, A.M., Salazar, G., Sanchez-Baracaldo, P., Lima-Mendez, G., Hingamp, P., Alberti, A., Sunagawa, S., Bork, P., de Vargas, C., Raes, J., Bowler, C., Wincker, P., Zehr, J.P., Gasol, J.M., Massana, R., Acinas, S.G.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Nature Publishing Group 2016
• Subjects: Cardiovascular and Metabolic Diseases; South Atlantic Ocean
• Online Access: http://edoc.mdc-berlin.de/15599/; https://edoc.mdc-berlin.de/15599/; http://edoc.mdc-berlin.de/15599/1/15599oa.pdf; https://doi.org/10.1038/ncomms11071

The unicellular cyanobacterium UCYN-A, one of the major contributors to nitrogen fixation in the open ocean, lives in symbiosis with single-celled phytoplankton. UCYN-A includes several closely related lineages whose partner fidelity, genome-wide expression and time of evolutionary divergence remain to be resolved. Here we detect and distinguish UCYN-A1 and UCYN-A2 lineages in symbiosis with two distinct prymnesiophyte partners in the South Atlantic Ocean. Both symbiotic systems are lineage specific and differ in the number of UCYN-A cells involved. Our analyses infer a streamlined genome expression towards nitrogen fixation in both UCYN-A lineages. Comparative genomics reveal a strong purifying selection in UCYN-A1 and UCYN-A2 with a diversification process ∼91 Myr ago, in the late Cretaceous, after the low-nutrient regime period occurred during the Jurassic. These findings suggest that UCYN-A diversified in a co-evolutionary process, wherein their prymnesiophyte partners acted as a barrier driving an allopatric speciation of extant UCYN-A lineages.